The invention relates to a process for increasing the availability of a global navigation system comprising several spacecraft, each of which transmits information to a terminal for the purpose of determining its position. The invention also relates to a terminal for determining a position while using a global navigation system.
In a global satellite navigation system, the precise determination of a specified position with respect to the earth requires both local and global integrity of information transmitted to the terminal by a plurality of satellites. On the one hand, integrity is the ability of the global navigation system to warn a user (i.e., the terminal), within a predefined time, when parts of the system should not be used for the intended purpose. On the other hand, integrity is also the trust a user can have in the reliability of the information which he receives from the navigation system. In particular, this relates to the accuracy of the information.
Warnings are required, for example, when individual satellite or navigation position determination signals have errors. Such errors occur, for example, when a navigation signal of a satellite was generated at the wrong time (so-called “clock or time correction error”) or was created at a faulty location (so-called “faulty satellite orbit”). These errors may influence the actual propagation time of a signal from the satellite to the terminal and may therefore exert a strong influence on the precision of the navigation.
In order to minimize a measuring error during position determination by the terminal, in the case of the known global Galileo navigation system, the information of all satellites from which the terminal receives information will be processed. This approach is based on the assumption that, as a result of a maximum number of measurements which can in each case be performed based on the information transmitted by the respective satellites, an error in the position determination can be minimized. Here, it is an existing secondary condition that at most six of the satellites are permitted to be critical satellites. A critical satellite is defined as a satellite whose information is necessary for position determination, in order to leave an integrity risk below a predefined threshold value (a so-called tolerable or allocated integrity risk). For this reason, the terminal has a function for determining the number of critical satellites in a terminal geometry. Terminal geometry (also: user geometry) refers to the taking-into-account of those satellites whose information is to be used for the position determination.
Because of the large number of satellites to be taken into account in practice (in the case of Galileo, as a rule, 11 satellites) and because of the predefined specification of a maximum of six allowed critical satellites, there is a superproportionally high degree of unavailability of its global navigation system in practice.
It is therefore an object of the present invention to provide a process for increasing the availability of a global navigation system which comprises several spacecraft each of which transmits information to a terminal for the purpose of determining its position. It is also an object of the present invention to provide a terminal which determines its position using a global navigation system.
These and other objects and advantages are achieved by the process according to the invention, in which information is in each case transmitted to the terminal by a plurality of spacecraft. From the plurality of spacecraft, a first subset (with at least one spacecraft) and a second subset are determined, with the second subset being constituted by those spacecraft which are not included in the first subset. An integrity risk is then determined based only on information transmitted by the second subset of spacecraft. The first and the second subsets of spacecraft are determined ultimately such that the integrity risk associated with the information transmitted by the second subset of spacecraft is optimized relative to the integrity risk associated with the information of all spacecraft included in the plurality of spacecraft.
The terminal according to the invention, which determines its position using a global navigation system, comprises devices for carrying out the process according to the invention.
The invention is based on a recognition that the accuracy of the position determination of the terminal does not depend on the number of available measurements which can in each case be carried out from the information transmitted by the respective spacecraft. On the contrary, a high degree of accuracy of the position determination can also be achieved from a smaller number of measurements. However, by using a smaller number of spacecraft for a position determination, and by optimizing the integrity risk, the availability of the navigation system can also be increased.
In this case, the optimization, on the one hand, takes into account the need to maintain the integrity risk below a predetermined tolerable value, and/or the need to maintain the number of critical spacecraft below a predefined maximum number of critical spacecraft, on the other hand. The optimization with respect to the integrity risk and/or the number of critical satellites can be achieved for a position determination by eliminating from consideration those spacecraft whose information results either in an increased integrity risk or in an increased number of critical spacecraft. Accordingly, in an optimization process, spacecraft (from among those from which the terminal receives information) are therefore allocated to the first subset (which should not be used for position determination), and to the second subset (based on which the position determination should finally take place). Only the information of the second subset of spacecraft is used to determine the position of the terminal. Thus, information from the first subset of spacecraft is not used for this purpose.
According to an embodiment of the invention, the first subset and the second subset of spacecraft are determined iteratively, so that the integrity risk determined from information transmitted by the second spacecraft, is minimized relative to the integrity risk that was determined from information of all spacecraft the plurality of spacecraft.
To minimize integrity risk, a number of spacecraft from the plurality of spacecraft are allocated to the first subset, and the integrity risk of the second spacecraft remaining in the second subset is determined. This step is repeated for all possible combinations of a first subset. In principle, the number of spacecraft may be arbitrary. It is advantageous to carry out first the iteration with the number “1”. Should this not result in any significant reduction of the integrity risk, the iteration can be repeated, for example, for a number “2”. This approach can be arbitrarily expanded. Those second spacecraft of the second subset where the integrity risk is minimal form the plurality of spacecraft for a next iteration step. The above-mentioned steps are repeated until a minimum integrity risk has been reached. By means of the above-mentioned process steps, those spacecraft are successively excluded from a measurement for determining the position of the terminal which contribute to the greatest reduction of the integrity risk.
In this case, a further embodiment of the invention determines whether the integrity risk resulting from the successive removal of at least one spacecraft is lower than a tolerable integrity risk. If so, the navigation system will be available.
According to a further embodiment, it is determined for each of the second spacecraft (of the second subset) whether it is a critical spacecraft. In this case, it is to be checked whether the number of critical spacecraft is greater than a number of allowed critical spacecraft. If so, it is attempted according to a further embodiment of the invention to reduce the number of the critical satellites to a tolerable number.
To minimize the number of critical spacecraft, a non-critical spacecraft from the determined (particularly, optimal) second subset of spacecraft is allocated to the first subset. This means that the non-critical spacecraft is at first excluded from the measurements. Then the number of critical spacecraft is determined within the remaining second subset. These steps are iteratively repeated until no more non-critical spacecraft can be determined in the second subset. This approach is based on the consideration that also the number of critical spacecraft may change positively as a result of the further removal of a non-critical spacecraft. If no other non-critical spacecraft can be removed from the second subset and if, after checking the number of the critical subset, the number of critical spacecraft has not fallen below a predefined value, the optimization will come to an end at this point.
The number of critical spacecraft is advantageously minimized when the number of critical spacecraft is greater than a maximum permitted number of critical spacecraft.
The invention further comprises a computer program product which can be loaded directly into the internal memory of a digital computer, including software code sections by means of which the steps of the process according to the invention can be carried out when the product is running on a computer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.